1. Field of the Invention
This invention relates to concrete marine floats and, more particularly, to a concrete marine float employing two varieties of concrete having differing characteristics.
2. Description of the Prior Art
Marine floats having a shell of standard aggregate concrete surrounding either a hollow or buoyant foam core are in common use. While these floats are generally capable of forming strong, long lasting and relatively stable marine piers, they are extremely heavy thereby making them expensive to transport to an installation site. Also, their heavy weight necessitates a relatively deep float in order to achieve the necessary freeboard so that the floats utilize a relatively large quantity of concrete and other materials thereby making them expensive to manufacture.
To alleviate the above described problems with concrete floats of standard aggregate concrete, marine floats have been manufactured of lightweight shale aggregate concrete which has almost the strength of standard aggregate concrete but is far less dense. Although these lighter floats effectively solve some of the problems associated with standard aggregate concrete, lightweight shale concrete is far more expensive and it is extremely energy intensive to produce. Furthermore, it has a greater tendency to absorb water.
An additional problem with marine floats of standard aggregate and expanded shale aggregate concrete results from the heavy weight of the concrete in combination with concrete's well known inability to withstand tensile loads. Since the standard aggregate concrete and the expanded shale aggregate concrete have a much greater density than water, gravity exerts a downward force on the bottom of the float which produces tension in the side walls of the float. The side walls are sometimes unable to withstand this tension causing the bottom to fall away from the float.
A third approach to the fabrication of concrete marine floats is the utilization of foam aggregate concrete. The manufacture and characteristics of foam aggregate concrete are fully described in Bagon et al "Marine Floating Concrete made with Polystyrene Expanded Beads, Magazine of Concrete Research, Vol. 28, No. 97, December 1976" and in U.S. Pat. Nos. 3,272,765 and 4,011,355. In this type of float the foam aggregate concrete is cast in solid blocks which are then secured to each other to form a pier. Although foam aggregate concrete is far lighter than even expanded shale concrete, it still has a density of 85% of the density of sea water thus requiring an excessively deep float to provide sufficient freeboard for pier construction. For example, a foam aggregate concrete float providing a standard 14 inch freeboard would be over 7 feet thick. The tremendous cost of this quantity of concrete plus enormous freight costs as well as the frequent lack of sufficient water depth for floats having this thickness preclude the widespread use of such floats.
An apparent solution to the above described limitation of foam aggregate concrete would be to utilize foam aggregate concrete to form a shell surrounding a hollow or buoyant foam core. However, foam aggregate concrete is much weaker than either standard aggregate concrete or lightweight shale concrete. This weakness manifests itself in an inability to withstand breaking up of the float responsive to stresses imparted by strong tidal action or vessels and in poor wearing qualities principally on deck walkways.
Thus serious problems and limitations must be resolved or at least compensated for in the construction of marine piers utilizing any of the presently available varieties of concrete marine floats.